As Apollo 11 slowly descended to the moon’s surface, the team of NASA scientists responsible for choosing the first lunar landing site was as anxious and in awe as the rest of the world.

“All our hearts were pounding,” recalled geologist Farouk El-Baz, a NASA scientist on the Apollo space program’s site-selection committee. “What if the moon was completely different than we thought? We were not 100 percent sure of all aspects, so there was room for error.”

The capsule, in fact, touched down July 20, 1969—4 miles from the predicted landing point and about a minute-and-a-half sooner than scheduled. But as commander Neil Armstrong proclaimed “the Eagle has landed,” made his historic “giant leap for mankind” and returned the spaceship safely to Earth, the mission—and five Apollo lunar landings that followed—was considered a rousing success.